1. Field of the Invention
The present invention relates to a recording head driving device for printers, and more particularly to a driver used for providing a half-tone of in image by changing the length of a dot within one pixel.
2. Description Related to the Related Art
Thermal printers, LCD light printers, and the like use a recording head having M recording elements (M being an integer) disposed in the main scan direction to record M dots on a recording sheet while moving the recording head or recording sheet in the sub scan direction. Thermal printers are used widely nowadays. A known recording head driving device for such thermal printers is structured as having a shift register for shifting sequentially inputted one bit serial signals and converting the one bit serial signals into parallel signals, M latches for latching parallel M signals outputted from the shift register, and M gates for performing a logical multiply between an output signal from each latch and a strobe signal and powering a corresponding thermal element.
Thermal printers used for printing binary images such as characters and lines assign one bit per one pixel. On the other hand, printers used for printing half-tone images of n tonal levels, such as direct imaging type thermal printers and sublimation transfer type thermal printers, assign N bits per one pixel (N being an integer). For example, in order to represent sixty-four tonal levels, eight bits per one pixel are assigned to image data to be expressed by a binary coded hexadecimal notation. A pre-processor converts 8-bit image data into serial 64 binary signals representing an on/off state of each thermal element, and sends the serial 64 binary signals one after another sixty-four times to a recording head driver. For a thermal element recording a maximum density pixel, 64 current pulses are supplied for generating a large thermal energy.
In order to provide a half-tone image, an area gradation method has been proposed. According to this method, one pixel is divided into n sub-lines. Each time a recording head or a recording sheet is moved by one sub-line amount in the sub scan direction, the power conduction time for each recording element is controlled by changing the length of each dot in n steps in the sub scan direction. This area gradation method is effective for a printer which cannot control the density of a dot itself, such as a thermal wax transfer type printer which heats the back of an ink film and transfer melts or softens ink onto a recording sheet.
Printers incorporating such an area gradation method may use the recording head driver described above. However, such a recording head driver transfers data one bit after another, necessitating n times of data transfer for one pixel. Therefore, the data transfer amount proportionally increases as tonal steps increase, which results in a low printing speed. In addition, it is necessary to provide a pre-processor for converting N-bit parallel image data into n serial image data and complicated peripheral circuits.